Transcriptional Regulation of Gene Expression at Early Stages of Arabidopsis Vein Development

• Author / Creator

  Gardiner, Jason L

• How multicellular organisms activate gene expression in the correct cells at the correct time is a central question in biology. In animals, in which this question has been investigated extensively, broadly expressed transcription factors activate target gene expression in narrow domains by a combination of differential affinity of such transcription factors for their binding sites in target genes and combinatorial interactions between transcription factors. In plants too, broadly expressed transcription factors often activate target gene expression in narrow domains; however, how broadly expressed plant transcription factors activate target gene expression in narrow domains is unclear. I addressed this question for the MONOPTEROS (MP)–ARABIDOPSIS THALIANA HOMEOBOX8 (ATHB8) pair of Arabidopsis genes. ATHB8 expression is activated in files of vascular precursor cells of the leaf. Activation of ATHB8 expression in such narrow domains depends on binding of the broadly expressed MP transcription factor to a low-affinity MP-binding site in the ATHB8 promoter. I tested the hypothesis that activation of ATHB8 expression is restricted to narrow domains by binding of peak levels of the broadly expressed MP to a low-affinity MP-binding site in the ATHB8 promoter. I found that activation of ATHB8 expression is restricted to narrow domains by an MP-dependent incoherent feed-forward loop: MP activates ATHB8 expression, but it also activates expression of the AUXIN/INDOLE-3-ACETIC ACID INDUCIBLE (AUX/IAA) gene IAA12/BODENLOS, which inhibits MP-mediated activation of ATHB8 expression. In animals, a similar regulatory mechanism is most frequently used to activate gene expression in narrow domains, suggesting conservation of regulatory logic of striped gene expression in animals and plants despite the independent evolution of their multicellularity. The development of multicellular organisms requires that not only do cells differentiate correctly but that they do so at the correct position. The correct differentiation of cells at the correct position depends on communication between cells; therefore, how cells communicate with one another is a key question in biology. In animals, such cell-cell communication often relies on direct coupling between cells. Direct cell-cell coupling is precluded in plants by a wall that surrounds each cell; yet, precisely because a cell wall holds plant cells in place and prevents their migration, positional signals, rather than lineage, specify cell fate in plants. For example, positional signals from leaf veins have long been known to control the differentiation of the adjacent bundle-sheath cells; however, the nature of such vein-derived signal had remained unclear. My results suggest that in Arabidopsis the SHORT-ROOT (SHR) transcription factor is such signal: the SHR gene is expressed in vascular precursor cells but the SHR protein is additionally localized to the adjacent layer of bundle-sheath cell precursors, and shr mutants fail to differentiate bundle-sheath cells. Available evidence suggests that restriction of SHR expression to vascular cells is critical to SHR function; yet what controls SHR vascular expression is poorly understood. I addressed this question for the Arabidopsis leaf. I found that SHR expression in vascular precursor cells is required for SHR-mediated induction of bundle-sheath differentiation, and that both SHR expression in vascular precursor cells and SHR function in bundle-sheath cell differentiation are directly and positively controlled by a group of previously functionally uncharacterized transcription factors of the DNA-BINDING WITH ONE FINGER (DOF) family. My results provide long-awaited molecular details of how veins act as source of positional signals that specify fate of adjacent cells—positional signals which are so critical for the development of multicellular organisms such as plants, the cells of which, unlike those of animals, are unable to migrate.

• Subjects / Keywords

  • Transcriptional Regulation
  • Differential Gene Expression
  • Arabidopsis
  • Vein Development

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3319S99F

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.